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Trialkoxysilane is used in various fields such as production of siliceous oligomer, monosilane, silicon for solar energy or semiconductor, and the like, and for synthesis of such trialkoxysilane, two basic methods are used.
First method is to synthesize trialkoxysilane in fixed bed or fluidized bed under a vapor-gas environment. The method for preparation in a vapor-gas environment is to pass alcohol vapor through a silicon powder layer including catalyst in fixed bed or fluidized bed. However, a fixed bed reactor is not widely-used because of the difficulty of maintenance of uniformly distributed temperature throughout the whole reactor volume. In case of direct synthesis in fluidized bed, such disadvantage is avoidable.
In [1], a method for diluting gas in alcohol vapor may be used to prevent or minimize that temperature in fluidized bed reaches peak. As a diluting gas, argon, nitrogen, helium, neon, hydrogen, and the like may be used. However, if additional material is applied, production cost, and also losses of trialkoxysilane and alcohol caused by the carryover of inert gas are increased. Meanwhile, the output power of desired trialkoxysilane is improved by synthesis under a lower pressure. In this case, conversion rate of silicon is 90%, selectivity of silicon is 84.2%, while under a normal pressure, 65%, and 48.8%, respectively. However, there is a disadvantage in that as pressure is lowered, reaction rate decreases, and as a result, productivity is deteriorated.
[2] suggests passing hydrogen with alcohol vapor through contact mass. If hydrogen is applied to the technical process of preparation of trialkoxysilane, additional preparing and purifying parts for hydrogen are needed, which increases preparation cost. When contact mass was activated with nitrogen, and zinc was added as an accelerator at reaction temperature of 280° C. or less, the content of triethoxysilane in reacting materials was 87%, but the conversion rate of silicon was very low, 23%.
It is preferred to stepwise carry out the activation of contact mass including silicon and a catalyst, at 450° C. or less in [1], or at 300-350° C. in [2] and [5], under a nitrogen or other inert gas atmosphere.
[6] to [9] suggest a method of applying hydrogen for silicon and catalyst activation. Activation using hydrogen is carried out at about 400° C. in fixed bed or fluidized bed. The mixture of silicon and catalyst contains 1.5% or more of copper. However, any information as to the resulting selectivity, reactivity, and reaction stability is not presented.
In [1] to [9], as a result of synthesis in fluidized bed for obtaining triethoxysilane, in case the reaction is carried out under atmospheric pressure, the yield of triethoxysilane and the conversion rate of silicon are not high, and in case the reaction is carried out under low pressure, main synthesis index is improved, but such advantage is offset by consequent technical features. In addition, in case liquefied material is additionally injected with alcohol, some numerical values increase, but some problems arise in the synthesis process of trialkoxysilane in fluidized bed, and minor carryover of silicon and catalyst necessarily occurs, thereby requiring an additional filtering process for final product.
Second method is to carry out a direct reaction between silicon and alcohol in suspension state in the liquid solvent environment of a reactor equipped with a stirring device. This method is recently widely used, because in case of using a solvent, the temperature of reaction mixture may be uniformly maintained to greatly reduce the possibility of overheating reaction environment, and prevent side reaction, thereby raising the selectivity of trialkoxysilane and the conversion rate of silicon.
In the synthesis of trialkoxysilane, the temperature is maintained high, up to 300° C., and thus, a solvent used in the synthesis should not be decomposed at such temperature. Solvent should effectively maintain uniform temperature in reaction system. In addition, it should not generate oxidation at reaction temperature ranging 100-300° C., as well as highly disperse the powder.
[10] and [11] suggest using alkylated benzene, and [12] suggests using alkylated naphthalene-“THERMINOL” oil. Details of high temperature solvents of THERMINOL® 59, 60, 66, DOWTHERM® HT, MARLOTHERM® S, MARLOTHERM® and other trademarks are described in [13] and [25].
[13], [14] and [15] suggest that the amount of solvent used in synthesis should be 1:2-4:1 of solvent:silicon, preferably 1:1-2:1.
In [16]-[20], it takes a considerable induction period to activate the reaction after pouring reaction raw materials of silicon and alcohol, which may last for 1 hour to 12 hours. The main reason for generation of induction period is an oxide film on the surface of silicon. In order to reduce induction period, it is suggested that activation step should be added in the synthesis process of trialkoxysilane.
[13] reviews the activation process in very detail. Activation may be carried out in the corresponding reactor in which the reaction proceeds or a separate equipment. It is preferred to move silicon activated in a separate equipment from dry neutral environment to the reactor. Activation is carried out at 20 to 400° C., preferably 150 to 300° C. It is suggested that hydrogen and nitrogen are used together as activating gas, and silicon is activated by methanol for the reaction with ethanol, because methanol has higher reaction activity to silicon than ethanol or higher alcohol. For example, if 5% of methanol is added to ethanol, reaction rate significantly increases. Herein, a reaction suspension containing 1 kg of silicon, 14.1 g of copper hydroxide, and 2.1 kg of solvent Marlotherm®S was activated at 150 to 250° C. for 65 minutes using hydrogen and nitrogen. Methanol was fed at 250° C. for 5 hours at a rate of 4.3 g/min. Thereafter, the temperature was lowered to 230° C., the feed of methanol was stopped, and ethanol was fed at the same rate, wherein the feed of hydrogen was stopped, and that of nitrogen was sustained. The total amount of activating material is stoichiometrically calculated, and enough to convert divalent or monovalent copper catalyst to free copper. It takes considerable time for an actual activity process, and it is asserted that it is caused by a large silicon-copper mass surface (fine particle diameter of silicon 50-300 μm). Meanwhile, a special condition as to granularity of used catalyst is required. Fine particle size should range 1-100 μm, preferably 0.1-50 μm, more preferably 0.1-30 μm. At the same time, specific surface area of the catalyst within a raw material is 0.1-2 m2/g, preferably 10-50 m2/g. Direct synthesis reaction of silicon using alcohol is feasible both in periodic mode and continuous mode. In periodic mode, all silicon is put into a reactor early in the process, while alcohol is continuously fed until all silicon is reacted. Depending on the output, it is also possible that a certain amount of silicon is fed in turn, and alcohol is continuously fed. In case of a reactor in continuous mode, only silicon or silicon containing a catalyst is added after starting process. In this case, the content of catalyst is minimum, or controlled so that side reaction decomposing alcohol does not occur. Reaction temperature is 150° C. or more, but not higher than the temperature where the decomposition of alcohol and solvent occurs. It is preferred to carry out the reaction at 200-260° C. In methanol reaction, 220-250° C. is preferred, and in ethanol reaction, 200-240° C. is preferred. It is possible to carry out the direct synthesis reaction of trialkoxysilane at both high pressure and low pressure, but atmospheric pressure is preferred.
[21] and [22] suggest treating powdered silicon with hydrogen fluoride in order to remove an oxide film on silicon surface prior to working for shortening induction period.
[10] and [22] suggest activating reaction mass by maintaining high temperature under a nitrogen, argon, and other inert environment, and [23] suggests pre-mixing silicon and a catalyst in mill for 8 hours in an inert atmospheric condition, respectively.
[21] suggests injecting alkyl chloride, hydrogen chloride, or ammonium chloride for activating silicon prior to synthesis, and [24] suggests injecting halides such as NH4HF2. However, in case materials such as halide, alkyl halide and methanol are injected to a reactor prior to synthesis, a distillation step of aimed material is added for removing impurities in aimed material, which means that the preparation technique of trialkoxysilane becomes complicated.
Therefore, it is appreciated that induction period in a direct synthesis process of trialkoxysilane is not simply understood, and effective solution therefor does not exist. In case of injecting an additional reagent in synthesis process, the reagent should be removed from the final product, which adds a further step, and consequently makes the preparation technique of trialkoxysilane complicated and its production cost high.
In [11], [13], [14], [17] and [21], the main synthesis reaction of trialkoxysilane is accompanied by a side reaction which may form oligoalkoxysiloxane, moisture and other side reaction products, and they may be gradually accumulated under reaction environment to lower reaction rate of synthesis process. In [14], the reason is that metal being in the form of impurity in the synthesis of silicon may be contained in a catalyst and used. Copper metal is accumulated in a solvent component as a result of decomposition of a catalyst. If silicon and trialkoxysilane containing such remaining silicon and impurities are accumulated, reaction rate decreases. In such case, in order to continuously use a solvent in synthesis process of trialkoxysilane, the solvent must be regenerated.
[16] suggests using aluminium (0.01-10%, preferably 0.1-2%), [2] suggests using zinc, [25] suggests using an organic or inorganic compound possessing at least one phosphorus-oxygen bond, as an accelerators of reaction forming trialkoxysilane for increasing efficiency of main synthesis index of trialkoxysilane. However, any information about the effects therefrom is not provided.
[1] suggests a process for producing trialkoxysilane comprising pulverizing silicon and performing interaction of ground silicon and alcohol under the action of a catalyst.
Industrial silicon ground to a particle size of 500 μm in the air is used as a feedstock. Ethanol and methanol are used as alcohol reagents, and a copper-containing compound such as copper (I) chloride (CuCl) is used as a catalyst. They are mixed with ground silicon, and heated at 300° C. or less for several hours, thereby activating the catalyst to activate technical interaction of alcohol and silicon. Such technical method is also used in other methods similar to that of [1]. Together with this, for the purpose of interaction of silicon and alcohol, an additional catalyst in the form of halides is applied, and as organic and inorganic materials containing a halogen component, chlorides, fluorides, methyl bromide, ethyl bromide, ethylene trichloride, hydrogen fluoride (HF), hydrogen chloride (HCl), hydrogen bromide (HBr), hydrogen iodide (HI), and the like are mentioned. The above method has significant disadvantages in spite of its usability. The most important thing among those is additional considerable energy loss caused by pre-heating treatment at 300° C. or less for a long period, thereby extending entire processing time, and increasing energy consumption. In addition, the technical measures to apply gas-type halides for activation of process is not environmentally safe.
[26] includes pulverizing process of silicon, and interaction process of alcohol and ground silicon through a catalyst in a heated solvent environment with a reagent activated. Silicon is milled to fine particles to a size of 500 μm in the air using a ball mill. As an alcohol reagent, ethanol and methanol are used, and triethoxysilane and trimethoxysilane are obtained as a final product. As a catalyst, usually a copper-containing compound, mainly CuCl is used. As a solvent, polyaromatic oil is used, and the main technical process of an interaction between milled silicon and alcohol is performed under the environment where the solvent is heated to 200° C. The activation technique for an reagent is applied, and the reason for applying such technique is as follows: In the preparation of trialkoxysilane according to the illustrated schematic, impurities in raw materials are accumulated in reaction mass to make the consumption of reaction mass uneven, and solvent is partly consumed in a side reaction caused by impurities in raw materials, thereby generating unreacted silicon. Therefore, a technique to activate reagent is applied, which bleeds and participates a reaction mixture suspension containing unreacted silicon, and adds an adequate amount of solvent and catalyst, to return the concentrated suspension to the process. Such process is carried out several times during technical processing as unreacted silicon is accumulated in a reactor in the form of deposits. Like other similar methods, in spite of decrease in side reaction, increase in product yield, decrease in raw material loss, and increase in equipment productivity, the method has a technical disadvantage of having very complicated process, because the process is activated by applied technical measures, that is, separating unreacted silicon, and periodically repeated adding concentrated mixture of solvent and catalyst to reaction mass, in order to supplement the loss of solvent and catalyst. Moreover, since it is possible to rapidly reduce or slow down the reaction in the addition of an amount of new reagent instead of bled part, any measures followed by remove or shortening of reaction induction period are not considered.